Screening arrangement

ABSTRACT

Apparatus for screening pulp suspensions containing heavy impurities is disclosed including a casing with an inlet for the pulp suspension at the lower end of the casing, a stationary screen mounted within the casing, a rotor mounted for rotation within the stationary screen thus forming a screening zone between the rotor and the stationary screen, with the inlet disposed within the rotor, the rotor including at least one opening adjacent to its upper end for transferring the pulp into the screening zone and a chamber for the heavy impurities disposed adjacent to either the upper or lower end of the casing, the rotor including strips or grooves for moving heavy impurities from the inner surface of the rotor to the chamber.

FIELD OF THE INVENTION

The present invention relates to an arrangement for screening pulpsuspensions in order to separate impurities and other pulp fractions,which are not desired to be included in the final product, such ascoarse particles, undefibered material and poorly worked fibers.

BACKGROUND OF THE INVENTION

During the manufacture of fiber suspensions, undesired coarse particles,such as, for example, undefibered material, bark, knots and the like,are present in the suspension as a result of incomplete manufacturingprocesses. In addition, other impurities, which are both light andheavy, such as plastics, sand and scrap, can also be found in thesuspension. In particular, fiber suspensions of slushed return fiberscontain great amounts of foreign impurities. Heavy impurities, such asstones, sand and glass, plaster and wire clips, and light impurities,such as certain plastics, agglomerated glue lumps ("stickies") etc., cancause interruptions in the screening process. It is, therefore, desiredto separate them at an early stage of the screening process. For thispurpose, special devices, for example knot screens or refiners, can bearranged before the screen in order to eliminate the coarse impuritiesor reduce their size. Light impurities can also be separated by specialdevices before the screen. It is also possible to separate the coarseimpurities by a first screening step in the screening arrangement or toseparate scrap and heavy particles when the suspension enters thescreen, and other impurities during the fine screening process. In thelast mentioned case, the screening process can be disturbed by theimpurities, as mentioned above.

During the screening of pulp suspensions it is also desired to have ahigh pulp concentration, for example, about 3 to 5%, and a low rejectdraw-off in order to achieve a high production capacity and to avoidunnecessarily large liquid transport in the screening system. Highconcentration and low reject draw-off, however, imply greaterdifficulties in separating the impurities from the pulp.

SUMMARY OF THE INVENTION

According to the present invention, the aforesaid problems are solvedfor designing the screening arrangement for screening the pulpsuspension in two integrated steps, where heavy and light impurities canbe separated in a first step, and fine screening takes place in a secondstep.

In accordance with the present invention, these objects have thus beenachieved by the discovery of apparatus for screening pulp suspensionscontaining heavy impurities comprising a casing including a lower endand an upper end and including an inlet for the pulp suspension disposedat the lower end of the casing, a stationary screen mounted within thecasing, a rotor mounted for rotation within the stationary screen, therotor including an inner surface, a lower end and an upper end andforming a screening zone between the rotor and stationary screen, theinlet being disposed within the rotor, the rotor including at least oneopening adjacent to the upper end of the casing for transferring thepulp into the screening zone, and a chamber for the heavy impuritiesdisposed adjacent to one of the upper and lower ends of the casing, therotor including heavy impurity movement means for moving the heavyimpurities from the inner surface of the rotor to the chamber.

In accordance with one embodiment of the apparatus of the presentinvention, the chamber for the heavy impurities is disposed at the lowerend of the casing adjacent to the lower end of the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the chamber for the heavy impurities is disposed at the upperend of the casing adjacent to the upper end of the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the rotor includes a cylindrical surface including strips forguiding the heavy impurities upwardly within the rotor. In a preferredembodiment, the strips are axially disposed along the inner surface ofthe rotor. In another embodiment the strips are disposed at apredetermined angle with respect to the axial direction of the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the rotor is cylindrical and the cylindrical inner surface ofthe rotor includes grooves for guiding the heavy impurities upwardlywithin the inner surface of the rotor. In another embodiment, thegrooves are axially disposed along the inner surface of the rotor. Inyet another embodiment, the grooves are disposed at a predeterminedangle with respect to the axial direction of the rotor along the innersurface of the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the strips extend in the form of a screw about the innersurface of the rotor. In a preferred embodiment, the grooves extend inthe form of a screw along the inner surface of the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the rotor is conical and the conical inner surface of therotor includes an inner diameter which increases in the downwarddirection.

In accordance with another embodiment of the apparatus of the presentinvention, the inlet is tangentially disposed with respect to the rotor.

In accordance with another embodiment of the apparatus of the presentinvention, the apparatus includes a stationary housing disposed withinthe rotor, the stationary housing including a rotationally symmetricalwall axially spaced from the inner surface of the rotor. In oneembodiment, the apparatus includes strips disposed on the rotationallysymmetrical wall of the stationary housing for discharging lightimpurities therealong. In another embodiment, the apparatus includesgrooves disposed along the rotationally symmetrical wall of thestationary housing for discharging light impurities therealong.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more fully understood with reference to thefollowing detailed description, which in turn refers to the Figures inwhich:

FIG. 1 is a side, elevational, sectional view of the screening apparatusof the present invention; and

FIG. 2 is a side, elevational, sectional view of another embodiment ofthe screening apparatus of the present invention.

DETAILED DESCRIPTION

The screening apparatus according to FIGS. 1 and 2 comprises an airtightcasing 1 with a stationary, preferably cylindrical screening member 2with a vertically symmetrical axis. Within the screening member 2, adrum-shaped rotor 3 is located, which extends along the entire screeningmember. The rotor 3 is concentric with the screening member 2, so thatan overall screening zone 4 is formed between the rotor and thescreening member. The rotor 3 is supported by a stationary housing 20,which is located within the rotor and has a rotationally symmetricalwall extending axially spaced from the inside of the rotor. This wallcan be cylindrical or conical.

An inject inlet 5 for the pulp is connected to the casing 1 for thesupply of pulp from below to the lower portion of the inside of therotor 3. The inlet 5 is preferably located tangentially, so that theinject is supplied in the direction of rotation of the rotor 3.

The rotor 3 is designed as a drum, through which the pulp suspension isintended to flow upward and through one or several openings 6 in theupper portion of the rotor 3 for transferring the pulp to the upper endof the screening zone 4. The rotor 3 is provided on its outside withpulsation generating means 7 extending into the screening zone 4.

In the embodiment shown in FIG. 1, the inside of the rotor is formed formoving heavy impurities downwardly to a chamber 8 in the lower portionof the rotor 3, and the lower edge of the rotor 3 extends downwardlyinto the chamber 8. In order to bring about this separation, the insideof the rotor can be conical, with the greatest diameter at its lowerend. The inner surface can be smooth, or it can possibly be providedwith strips or grooves extending axially or angularly with respect tothe axial direction. This angle can be small, or such that the stripsextend in the form of a screw about the rotor. When strips or groovesare used, the angle of the cone can be smaller. It is also possible toform the inside of the rotor cylindrical. The stationary wall of thehousing 20 can be formed in a corresponding manner for an upwarddischarge of light impurities.

In both embodiments shown an inlet 9 for dilution liquid is connected tothe casing 1 of the screening arrangement. This inlet communicates witha space 10 in the rotor 3, which space is formed with openings 11 in therotor for the supply of dilution liquid to the screening zone 4,preferably in the lower portion of the screening zone.

In the embodiment shown in FIG. 1, between the inlet 9 for dilutionliquid and the chamber 8, a passage 12 is located, through which arestricted amount of dilution liquid can pass. This passage 12 ispreferably provided between the lower edge of the rotor 3 and the upperdefining wall of the chamber 8.

The chamber 8 can be designed for discontinuous or continuous emptying,depending on the expected content of heavy impurities in the pulp.

In the embodiment shown in FIG. 1, pulp to be screened is suppliedthrough the inlet 5 to the inside of rotor 3. At the same time as thepulp is rotated by the rotor, it flows upwardly through the rotor to theopenings 6 at the top of the rotor. Owing to the effect of centrifugalforce, heavy impurities are collected adjacent the inner surface of therotor. Due to the rotor design, these impurities are guided downwardlyto the chamber 8, from where they can be removed in a suitable manner,as mentioned above. In this way, separation of heavy impurities isachieved in a first step.

In the embodiment shown in FIG. 2, the inside of the rotor 3 preferablyis cylindrical and can possibly be provided with strips 15 on groovesaxial with or at a small angle to the axial direction, so that the heavyimpurities are guided with the pulp flow upwardly along the inside ofthe rotor 3 to be discharged to a chamber 13 intended for this purposeat the upper edge of the rotor 3. According to this embodiment, theopenings 6 in the upper portion of the rotor 3 for transferring the pulpto the upper end of the screening zone 4 are formed with an edge 14extending a distance inwardly from the inside of the rotor 3. In thismanner, the coarse and heavy impurities are prevented from movingupwardly along the inside of the rotor to follow along with the pulpthrough the openings 6. These impurities, instead, are guided past theopenings 6 to the chamber 13, from which they can be taken outdiscontinuously or continuously, depending on the expected content ofimpurities in the pulp.

At the same time as heavy impurities are concentrated at the innersurface of the rotor 3, light impurities are concentrated at the wallsurface of the housing 20. These light impurities are guided upwardlyand accumulate centrally upwardly in the rotor 3, from where they can bedischarged. The light impurities, for example, can be guided upwardlythrough the rotor top and be discharged centrally form the upper portionof the casing 1. For this purpose, the wall of the housing 20 can beformed with strips or grooves in order to promote separation of thelight impurities. The separation of heavy and, respectively, lightimpurities, thus, takes place before the pulp enters the screening zone4.

The pulp flow flowing through the openings 6 at the top of the rotor 3continues downwardly in the screening zone 4 for fine screening in asecond step whereby the pulp is divided into accept and reject portions.This dividing of the pulp into accept and reject portions, is promotedby the pulsation generating means 7, which brings about pressure andspeed variations in the pulp suspension which are favorable for thescreening. Due to the accept portions passing through the screeningmember together with a portion of the liquid, the liquid content in thereject transported along the screening zone 4 decreases. This thickeningof the reject is counteracted by the supply of dilution liquid throughthe openings 11 in the rotor 3 at the end of the screening zone. Thesupply of dilution liquid is preferably controlled so that he outgoingreject has the desired concentration.

For removing the accept portion, an accept outlet 16 is connected to aspace 17 in the casing 1, which space is located outside the screeningmember 2. A reject outlet 18 is connected to the casing 1 fordischarging the reject portion after the screening zone 4.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

I claim:
 1. Apparatus for screening pulp suspensions containing heavyimpurities comprising a casing including a lower end and an upper endand including an inlet for said pulp suspension disposed at said lowerend of said casing, a stationary screen mounted within said casing, arotor mounted for rotation within said stationary screen, said rotorincluding an inner surface, a lower end and an upper end and forming ascreening zone between said rotor and said stationary screen, said inletbeing disposed within said rotor, said rotor including at least oneopening adjacent to said upper end of said casing for transferring saidpulp into said screening zone, and a chamber for said heavy impuritiesdisposed adjacent to one of said upper and lower ends of said casing,said rotor including heavy impurity movement means for moving said heavyimpurities from said inner surface of said rotor to said chamber.
 2. Theapparatus of claim 1 wherein said chamber for said heavy impurities isdisposed at said lower end of said casing adjacent to said lower end ofsaid rotor.
 3. The apparatus of claim 1 wherein said chamber for saidheavy impurities is disposed at said upper end of said casing adjacentto said upper end of said rotor.
 4. The apparatus of claim 1 whereinsaid rotor includes a cylindrical surface including strips for guidingsaid heavy impurities upwardly within said rotor.
 5. The apparatus ofclaim 4 wherein said strips are axially disposed along said innersurface of said rotor.
 6. The apparatus of claim 4 wherein said stripsare disposed at a predetermined angle with respect to said axialdirection of said rotor.
 7. The apparatus of claim 1 wherein said rotoris cylindrical and said cylindrical inner surface of said rotor includesgrooves for guiding said heavy impurities upwardly within said innersurface of said rotor.
 8. The apparatus of claim 7 wherein said groovesare axially disposed along said inner surface of said rotor.
 9. Theapparatus of claim 7 wherein said grooves are disposed at apredetermined angle with respect to the axial direction of said rotoralong said inner surface of said rotor.
 10. The apparatus of claim 4wherein said strips extend in the form of a screw about said innersurface of said rotor.
 11. The apparatus of claim 7 wherein said groovesextend in the form of a screw along said inner surface of said rotor.12. The apparatus of claim 1 wherein said rotor is conical and saidconical inner surface of said rotor includes an inner diameter whichincreases in the downward direction.
 13. The apparatus of claim 1wherein said inlet is tangentially disposed with respect to said rotor.14. The apparatus of claim 1 including a stationary housing disposedwithin said rotor, said stationary housing including a rotationallysymmetrical wall axially spaced from said inner surface of said rotor.15. The apparatus of claim 14 including strips disposed on saidrotationally symmetrical wall of said stationary housing for discharginglight impurities therealong.
 16. The apparatus of claim 14 includinggrooves disposed along said rotationally symmetrical wall of saidstationary housing for discharging light impurities therealong.